I needed a logo and social media designs for a blog. I had a good idea on the style of the logo I wanted, the colors I liked, and a list of ideas for the designers to toy with.
I've gone through the 99design process once before. Things went well this time, I received a lot of design submissions and many of the designers took my initial ideas and created some interesting work from them.
The only problem I can speak of is this: when I asked designers to move away from ideas they submitted early on, many of them stayed too attached to their initial works and just slightly reworked the same ideas over and over again. When I asked them to explore new ideas they had a tough time breaking out of the "constraints" they assumed or imposed on themselves from their own designers or other designers.
Outside of that, everything went very well. You find the better designers early on not by seeing who submits the design closest to what's in your head, but seeing who submits multiple well-done designs and are most responsive to your feedback.